Understanding how plants affect the soils around their roots has been studied for many years, but getting down to the nitty gritty to see the actual microbial activity they engender takes a more complicated approach. A new project, coordinated at Los Alamos National Laboratory in conjunction with Pacific Northwest National Laboratory, will allow us to understand the close relationship between a well-studied grass and the soil in which it grows. This could, in turn, result in more efficiency in projects such as biofuel production, underground carbon sequestration and soil fertility.
Around each plant’s roots, a thriving community of fungus and bacteria lives, called the rhizosphere, feeding off the organic acids, sugars and other compounds produced by the roots. That collaborative environment increases the nutrient and carbon cycling in the soil, which is helpful to the plant. That same collaboration can also reduce pathogens in the rhizosphere, protecting the plant. It turns out, variations in the plant-root secretions can greatly influence the bacterial/fungal populations that associate with the plant roots.
The unique composition of the compounds excreted by plant roots can act as a “recruitment tool” plants use to customize the root-associated microbial community. It is unclear yet which individual soil microbes are attracted by specific compounds and how the plant uses these compounds to attract beneficial microbial interactions. Some microbes may be “specialists” that are recruited by very specific secretions, called exudates, whereas others may be “generalists” and attracted by many different compounds.
Our work will focus on a type of grass called Brachypodium distachyon that is viewed as valuable for biofuel biomass, food, feed and forage. Unlike the grasses grown as crops, B. distachyon is well suited to experimental manipulation in the laboratory because of its small size, compact genome, having two sets of chromosomes, self-compatibility, rapid generation time and simple growth requirements.
In this study, we are focusing on oxalate, a small molecule found in several plants either as a root secretion or as an accumulation in the plant tissues. It is the cause of a very bitter taste in edible plants, such as in raw rhubarb, where apparently the bitter taste is a mechanism for deterring grazing animals from eating it.
Just a few of these plants will be grown in Los Alamos’ Health Research Laboratory facility for preliminary stages of the experiments; in later stages, we will use the facilities at the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory. These will allow more advanced growth conditions such as manipulating the growth-chamber environment. There, we can use stable, isotopically labeled carbon dioxide to track how CO2 taken up by the plant is converted into compounds that are excreted by the roots to attract and feed soil microbial populations. The microbes the plant roots have encouraged will convert these plant-produced compounds into a soil carbon pool that has long-term stability, effectively leading to atmospheric CO2 sequestration in soils, which keeps the harmful greenhouse gas from escaping into the atmosphere.
Interestingly, for this project we will be utilizing a native soil from New Mexico collected at the Sevilleta Long Term Ecological Site. We have preliminary data indicating organisms in these soils can participate in oxalate-mediated carbon sequestration. The Sevilleta soils can be connected with a lot of metadata that has been collected for many years, such as annual climate variables, including temperature and precipitation, that can help in designing future and larger-scale field experiments. These soils represent marginal lands, which we want to better understand for future land-use optimization for plant-based or plant-derived resource generation.
This project is funded for two years and will build on previous and ongoing investigations here at the lab on how the root microbiome influences plant physiology and soil functions. Basically, we are enlisting plants and their surrounding soil in the battle against climate change — and hoping to find a potential protection for the Earth in the earth itself.
Buck Hanson is a microbiologist in Los Alamos National Laboratory’s Microbial and Biome Sciences group. This project is funded under the U.S. Department of Energy’s Office of Science, as part of the Science Focus Area: Bacterial-Fungal Interactions and Their Role in Soil Functioning.
